This invention relates to an abrasive incorporated in magnetic recording media such as magnetic tape, magnetic disc, and the like, and to a magnetic recording medium comprising an abrasive (note: said abrasive is sometimes called a reinforcing agent, because it acts as an abrasive upon a magnetic head, while it serves as reinforcing agent for a magnetic tape or the like). More particularly, this invention relates to an abrasive for use in a magnetic recording medium such as magnetic tape, magnetic sheet, magnetic disc or the like to give excellent wear resistance to the magnetic recording medium without losing surface smoothness of the magnetic layer thereof and to give moderate abrasion to a magnetic head to renew the head as well, and relates also to a magnetic recording medium comprising such an abrasive.
A magnetic recording medium such as magnetic tape, magnetic sheet, or magnetic disc used in audio or video equipments or in computers comes in sliding contact at a high relative speed with a magnetic head at the time of recording and reproducing and, as a result, tends to become worn away. Conventional magnetic layers are unsatisfactory in this respect and, hence, a magnetic layer less susceptible to abrasion and of improved durability is demanded. Particularly in the case of a magnetic video recording medium, it is required to improve the so-called still picture reproducing characteristics (still characteristics), wherein recorded signals are reproduced by repeatedly scanning the same part of a magnetic coating with a revolving magnetic head.
To improve the above disadvantages, it has heretofore been proposed to incorporate as a reinforcing agent a nonmagnetic powder material such as chromia (Cr.sub.2 O.sub.3), alumina (Al.sub.2 O.sub.3), or silicon carbide (SiC) into the magnetic layer. It is not easy, however, to disperse uniformly the above reinforcing agents in the magnetic layer, and there occurs frequently non-uniformity of the dispersion, resulting in a decrease in wear resistance of the magnetic layer, an excessive wearing of the magnetic head, or a decrease in smoothness of the tape surface. To overcome such disadvantages, there is known a method in which the coating composition to be used in forming the magnetic layer is admixed with a dispersing agent such as n-butyl stearate, liquid paraffin, or sodium alkylbenzenesulfonate in its manufacturing stage and thoroughly mixed in wet state to improve the dispersibility of reinforcing agents. This method, however, is not effective for improving sufficiently the dispersibility.
The present inventors carried out an extensive study to overcome the above difficulties and, as a result, found that it became possible to improve the dispersibility more effectively, as compared with the known methods, by imparting the dispersibility to a reinforcing agent by a specific method and that by using such a reinforcing agent it became possible to improve the wear resistance of both the magnetic layer and the magnetic head and to improve also the smoothness of the tape surface. This invention has been accomplished based on the above finding.